Polycrystalline superhard material

ABSTRACT

A method for making polycrystalline superhard material comprises providing an electrically conductive substrate defining at least one deposition surface, electrophoretically depositing charged superhard particles or grains on to the deposition surface(s) of the substrate to form a pre-sinter body, and subjecting the pre-sinter body to a temperature and pressure at which the superhard material is thermodynamically stable, sintering and forming polycrystalline superhard material. There is also disclosed a superhard wear element comprising a polycrystalline superhard material produced by such a method.

FIELD

This disclosure relates to a method of making polycrystalline superhardmaterial, particularly but not exclusively to a method of makingpolycrystalline diamond (PCD) material, and to a method of makingelements comprising same.

BACKGROUND

Cutter inserts for machine and other tools may comprise a layer ofpolycrystalline diamond (PCD) or polycrystalline cubic boron nitride(PCBN) bonded to a cemented carbide substrate. PCD and PCBN are examplesof superhard material, also called superabrasive material, which have ahardness value substantially greater than that of cemented tungstencarbide.

Components comprising PCD are used in a wide variety of tools forcutting, machining, drilling or degrading hard or abrasive materialssuch as rock, metal, ceramics, composites and wood-containing materials.PCD comprises a mass of substantially inter-grown diamond grains forminga skeletal mass, which defines interstices between the diamond grains.PCD material comprises at least about 80 volume % of diamond and may bemade by subjecting an aggregated mass of diamond grains to an ultra-highpressure of greater than about 5 GPa and temperature of at least about1,200 degrees centigrade in the presence of a sintering aid, alsoreferred to as a catalyst material for diamond. Catalyst material fordiamond is understood to be material that is capable of promoting directinter-growth of diamond grains at a pressure and temperature conditionat which diamond is thermodynamically more stable than graphite. Somecatalyst materials for diamond may promote the conversion of diamond tographite at ambient pressure, particularly at elevated temperatures.Examples of catalyst materials for diamond are cobalt, iron, nickel andcertain alloys including any of these. PCD may be formed on acobalt-cemented tungsten carbide substrate, which may provide a sourceof cobalt catalyst material for the PCD. The interstices within PCDmaterial may at least partly be filled with the catalyst material.

Components comprising PCBN are used principally for machining metals.PCBN material comprises a sintered mass of cubic boron nitride (cBN)grains. The cBN content of PCBN materials may be at least about 40volume %. When the cBN content in the PCBN is at least about 70 volume %there may be substantial direct contact among the cBN grains. When thecBN content is in the range from about 40 volume % to about 60 volume %of the compact, then the extent of direct contact among the cBN grainsis limited. PCBN may be made by subjecting a mass of cBN grains togetherwith a powdered matrix phase, to a temperature and pressure at which thecBN is thermodynamically more stable than the hexagonal form of boronnitride, hBN. PCBN is much less wear resistant than PCD, which may limitits scope of application.

SUMMARY

Viewed from a first aspect there is provided a method for makingpolycrystalline superhard material, the method comprising providing anelectrically conductive substrate defining at least one depositionsurface, electrophoretically depositing charged superhard particles orgrains on to the deposition surface(s) of the substrate to form apre-sinter body, and subjecting the pre-sinter body to a temperature andpressure at which the superhard material is thermodynamically stable,sintering and forming polycrystalline superhard material.

In some embodiments, the substrate forms the cathode of anelectrophoretic cell apparatus, the superhard particles or grains beingsuspended in a liquid in contact with the deposition surface(s) and ananode, the superhard particles or grains being positively charged so asto be deposited on a deposition surface or surfaces of the substrateupon application of an electric potential between the cathodic substrateand the anode.

In some embodiments, the anode defines a complimentary surface orsurfaces opposing the deposition surface(s) of the cathodic substrate.

In some embodiments, charged superhard particles or grains are depositedon the substrate in a series of layers or strata.

In some embodiments, the deposition surface(s) of the substrate is/aremasked in certain areas or regions, the superhard particles or grainsbeing deposited on exposed portions of the deposition surface(s) so toform discrete three dimensional polycrystalline superhard structures.

The various layers or three dimensional structures of polycrystallinesuperhard material, in some embodiments, have at least one differentstructural characteristic, non-limiting examples of which may includemean superhard grain size, superhard grain content, and content ofcatalyst for diamond.

In one embodiment, the polycrystalline superhard material is PCD and thesuperhard particles or grains comprise diamond.

In some embodiments, the diamond particles or grains have an averageparticle or grain size of from about 5 nanometres to about 50 microns.

In some embodiments, a multimodal mixture of diamond particles or grainsof varying average particle or grain size are deposited on thesubstrate.

In some embodiments, the diamond particles or grains are pre-treatedwith hydrogen or a source of hydrogen ions to render them positivelycharged.

In some embodiments, additional particulate materials are added to thedispersed diamond particles or grains in order to be deposited with thediamond particles or grains on the substrate. Exemplary particulatematerials include any compounds containing elements from Groups IA-VIIIAand Groups IB-VIIIB, for example alkali or alkali earth metal, metal ornon-metal carbides, nitrides, oxides, carbonitrides, halides, borides,sulphates, phosphates, tungstates and the like, or of the unreactedelements, for example metal powders.

In some embodiments, the polycrystalline superhard material is PCBN andthe superhard particles or grains comprise cBN.

In some embodiments, the superhard grain content of the polycrystallinesuperhard material is at least about 80 percent, at least about 88percent, at least about 90 percent, at least about 92 percent or even atleast about 96 percent of the volume of the polycrystalline superhardmaterial. In one embodiment, the superhard grain content of thepolycrystalline superhard material is at most about 98 percent of thevolume of the polycrystalline superhard material.

In some embodiments, the polycrystalline superhard material is PCDmaterial that comprises a catalyst material for diamond, the content ofthe catalyst material being at most about 10 volume percent, at mostabout 8 volume percent, or even at most about 4 volume percent of thePCD material. In one embodiment, the PCD material comprises at least aregion that is substantially free of catalyst material for diamond.

In one embodiment, the pre-sinter body comprises deposited diamondparticles or grains and the method includes subjecting the pre-sinterbody in the presence of a catalyst material for diamond to a pressureand temperature at which diamond is more thermally stable than graphite.In one embodiment, the pressure is at least about 5.5 GPa and thetemperature is at least about 1,250 degrees centigrade.

In one embodiment, the precursor body comprises deposited cBN particlesor grains and the method includes subjecting the pre-sinter body to apressure and temperature at which cBN is more thermally stable thanhexagonal boron nitride (hBN). In one embodiment, the pressure is atleast about 2 GPa and the temperature is at least about 900 degreescentigrade.

Viewed from a second aspect there is provided a superhard wear elementcomprising an embodiment of polycrystalline superhard material made bythe above-described method.

In some embodiments, the superhard wear element comprises a plurality ofregions, each region comprising polycrystalline superhard materialhaving at least one different structural characteristic, non-limitingexamples of which may include mean superhard grain size, superhard graincontent, and content of catalyst for diamond. In some embodiments, atleast some of the regions are in the form of layers or strata. In oneembodiment, at least one of the regions is lean or substantially free ofmetallic catalyst material for diamond. In other embodiments, at leastsome of the regions are in the form of discrete three dimensionalpolycrystalline superhard structures.

In one embodiment, the superhard wear element comprises a structurecomprising polycrystalline superhard material joined to a substratecomprising cemented carbide material.

DRAWINGS

Non-limiting embodiments will now be described by way of example andwith reference to the accompanying drawings in which:

FIG. 1 is a schematic side view of an electrophoretic cell apparatus foruse in an embodiment;

FIG. 2A is a schematic drawing of the microstructure of an elementcomprising polycrystalline superhard material made by an embodiment;

FIG. 2B is a schematic drawing of the microstructure of an elementcomprising polycrystalline superhard material made by anotherembodiment;

FIG. 2C is a schematic drawing of the microstructure of an elementcomprising polycrystalline superhard material made by a furtherembodiment; and

FIG. 3 is an SEM micrograph of a cross-section of a sintered elementcomprising polycrystalline superhard material made by an embodiment.

DETAILED DESCRIPTION OF EMBODIMENTS

As used herein, a superhard material is a material having a Vickershardness of at least about 28 GPa. Diamond, cubic boron nitride (cBN),polycrystalline diamond (PCD) and polycrystalline cubic boron nitride(PCBN) material are examples of superhard material.

As used herein, polycrystalline diamond (PCD) material comprises a massof diamond grains, a substantial portion of which are directlyinter-bonded with each other and in which the content of diamond is atleast about 80 volume percent of the material. In one embodiment of PCDmaterial, interstices between the diamond grains may at least partly befilled with a binder material comprising a catalyst for diamond. As usedherein, “interstices” or “interstitial regions” are regions between thediamond grains of PCD material. In embodiments of PCD material,interstices or interstitial regions may substantially or partially befilled with a material other than diamond, or they may substantially beempty. Embodiments of PCD material may comprise at least a region fromwhich catalyst material has been removed from the interstices, leavinginterstitial voids between the diamond grains.

As used herein, polycrystalline cubic boron nitride (PCBN) materialrefers to a type of superhard material comprising grains of cubic boronnitride (cBN) dispersed within a matrix comprising metal or ceramic.PCBN is an example of a superhard material.

As used herein, a superhard wear element is an element comprising asuperhard material and is for use in a wear application, such asdegrading, boring into, cutting or machining a workpiece or bodycomprising a hard or abrasive material.

As used herein, a multimodal size distribution of a mass of grainsincludes more than one peak, or that can be resolved into asuperposition of more than one size distribution each having a singlepeak, each peak corresponding to a respective “mode”. Multimodalpolycrystalline bodies are typically made by providing more than onesource of a plurality of grains, each source comprising grains having asubstantially different average size, and blending together the grains.

Electrophoretic deposition (EPD) is a method of forming layers ofdeposited particles whereby electrically charged particles in liquiddispersion are migrated to one or other electrode by applying anelectric potential across two electrodes. Control of the properties ofthe layers may be achieved by controlling the magnitude and duration ofthe applied potential, the size and concentration of the suspendedparticles, and the relative orientation of the two electrodes.

With reference to FIG. 1, an embodiment of a method for making anexample embodiment of a polycrystalline superhard material is carriedout in an electrophoretic cell apparatus 10. The method includes placinga substrate 12, which forms the cathode of the electrophoretic cell 10,with a deposition surface 14 thereof facing towards an anode 16. Verydifferent configurations of the cathodic substrate 12 and anode 16 ofthe electrophoretic cell could be used. For example, they may bearranged to lie horizontally with one above the other, or they may becylindrically co-axial. In this embodiment, the cathodic substrate 12 ispartially encased in a rubber sleeve 18 such that only depositionsurface 14 is exposed. Furthermore, more than one surface, or analternative surface, may be exposed for providing a number of differentdeposition surfaces. The exposed deposition surface 14 and at least aportion of the anode 16 is immersed in a stable aqueous suspension 20containing superhard particles or granules 22, which have beenpre-treated in hydrogen to render them positively charged. A magneticstirrer 24 is used to maintain the superhard particles or granules 22 insuspension. In an example embodiment, a DC potential of about 3V may beapplied for about 2 minutes to generate the electric field between thecathode 12 and anode 16 to deposit the superhard particles or grainsonto the deposition surface 14 of the cathodic substrate 12, to form alayer 26 of deposited superhard particles or grains. The processconditions, such as the acidity of the suspension, the type and quantityof dispersants and the superhard particle or grain surface chemistry maybe optimised. The resulting pre-sinter body comprising the superhardlayer 26 deposited on the substrate 12 is then removed from the cell 10,dried and sintered.

Referring to FIGS. 2A to 2C, various configurations of superhardelements are shown. In its simplest form, FIG. 2A shows a superhardelement 30 comprising a single layer 32 of polycrystalline superhardmaterial, in this case polycrystalline diamond, bonded to a substrate34, in this case a Co—WC substrate. FIG. 2B shows a superhard element 40comprising a plurality of layers 42 of polycrystalline superhardmaterial, in this case polycrystalline diamond having varyingcharacteristic features, bonded to a substrate 44, in this case a Co—WCsubstrate. FIG. 2C shows a superhard element 30 similar to that of FIG.2A, except that in this case a non-planar interface 36 is definedbetween the layer 32 of polycrystalline diamond and the Co—WC substrate34.

In embodiments where the precursor body comprises diamond grains, thepre-sinter body may be sintered by subjecting it in the presence of acatalyst material for diamond to a temperature of about 1,350 degreescentigrade and a pressure of about 5.5 GPa to form an embodiment of PCDmaterial.

An ion permeable membrane may be placed between the anode and thecathodic substrate in the electrophoretic cell. This may allow ions topass through it between the anode and the substrate, while shielding thesubstrate from gas that may evolve at the anode surface, which maydisrupt the electrophoretic deposition process and reduce the efficiencyof electrophoretic deposition of the superhard particles or grains on tothe substrate.

The liquid used to disperse the powders may consist of organic orinorganic components, or a mixture of the two, for example water,alcohol, or a water-alcohol mixture.

The concentration of the dispersed particles or grains in the liquid mayvary from 0.1% to 60%, but preferably 10-20% by weight. Theconcentration may be maintained at a constant value by gradual orperiodic addition of extra particles or grains to the liquid for theduration of the deposition. Alternatively, the concentration may bevaried by not replenishing the particles or grains during deposition, sothat the concentration of dispersed particles or grains graduallydecreases, or the concentration may be increased by adding moreparticles or grains than has been removed due to deposition. Particlesor grains of various compositions and particle sizes may be added atdifferent times to obtain layers of varying composition and particlesize. Alternatively, the electrode assembly may be dipped in onesuspension for a certain amount of time to achieve deposition, thenlifted up and dipped in a different suspension to achieve deposition ofa different composition. This sequence may be repeated many times and indifferent suspensions to build up the required green-body deposit.

The substrate which serves as the electrode onto which the diamondparticles or grains or other superhard material is deposited, may be ofany material, as long as it is electrically conductive, e.g.electrically conductive polymers such as polypyrrole, or electricallyconductive composites e.g. graphite particles or fibres in polymer, orCo—WC. In one embodiment, the substrate comprises a composite of Co—WCand a layer of polycrystalline superhard material, the layer ofsuperhard material defining the deposition surface(s).

The substrate/electrode surface may be planar, non-planar (shaped), andmay be of regular or irregular shape, symmetrical or non-symmetrical.The substrate may vary in thickness from 0.1 millimetres to 10centimetres, and the diameter may vary from 0.1 millimetres to 10centimetres.

The counterelectrode or anode may consist of any electrically conductingmaterial, e.g. metal, graphite or coated metal or graphite. It may havea flat surface, or a shaped non-planar surface, which may becomplementary in shape to the deposition surface or surfaces of thecathodic substrate. The reason for this is that the current density atany region on the electrode, and therefore the amount of particles orgrains deposited at that region, is a function of the distance betweenthe two electrodes at that point. Regions on the substrate/electrodewhere it is closer to the anode/counterelectrode will experience afaster rate of deposition, and regions where it is further from theanode/counterelectrode will experience a slower rate of deposition. Bycontrolling the shapes of the substrate/electrode and theanode/counterelectrode, the shape of the surface of the deposited layer,i.e. the thickness of the layer at specific places, may be controlled.

The counterelectrode or anode may also be changed at various stages ofthe deposition process in order to tailor the deposited layer. Forexample, a counterelectrode shaped with a dimple or indentation in thecentre will cause more electrodeposition to occur on the periphery ofthe substrate/electrode, or alternatively a counterelectrode with aprotrusion in the centre will cause more electrodeposition in the centreof the substrate/electrode. The substrate/electrode may also be blankedoff in certain areas using a masking agent, causing the electrodepositedlayer to form around these blanked off areas. In this way, by using amodified counterelectrode or substrate/electrode at certain stages ofthe EPD process, the deposited layer may be tailor-made or functionallygraded according to requirements.

Generally the thickness of the electrodeposited layer may vary from 50nanometres to 5 millimetres.

Although the use of EPD in diamond synthesis has found application inthe coating of objects with diamond as separate particles, as discretenucleation sites for further diamond growth by chemical vapourdeposition, or as particles included in metal layers, it has not beenused for preparing green bodies for the high-pressure high-temperatureproduction of sintered polycrystalline diamond (PCD).

Thus, DE2011966 discloses the coating by EPD of an electricallyconductive support with non-metallic particles such as carbides,nitrides, borides, silicides and oxides of metals such as W, Mo, Ta, Nb,Ti, Zr; carbides and oxides of Si and B, diamond powder and Al₂O₃. Theproblem of adhesion to the support is overcome by firstelectrodepositing a metal layer to the support, followed byelectrophoretic deposition of the non-metallic powder such that thepowder particles lodge in the pores of the metal layer. The resultingproduct is a support coated with a layer of metal containing discrete,non-intergrown hard and wear resistant particles.

Layers of diamond film on silicon find wide application in thesemiconductor industry. However, such diamond films, grown by chemicalvapour deposition, lack adhesion and uniformity when grown on siliconsubstrates. U.S. Pat. No. 5,128,006 discloses the use of EPD to obtaindiscrete and adherent diamond particles on an oxidised siliconsubstrate.

The problem of adherence of diamond films to WC—Co substrates isaddressed in JP2002338386, which discloses a method of obtaining adiamond coating on a WC—Co substrate by first acid treating the WC—Cosubstrate, followed by EPD to deposit discrete diamond particles asseeds onto the acid-treated WC—Co surface, heat treating the seededsubstrate and finally growing a diamond film onto the diamond seeds bychemical vapour deposition.

U.S. Pat. No. 6,258,237 discloses a method of depositing diamondparticles on a surface of a substrate, the method comprising the stepsof (a) charging the diamond particles by a positive charge to obtainpositively charged diamond particles; and (b) electrophoreticallydepositing the positively charged diamond particles on the surface ofthe substrate, for obtaining a green diamond particles coat on thesurface of the substrate.

A different set of problems arise when manufacturing polycrystallinediamond (PCD) cutters, which are used mainly in oil and gas drillingapplications. During PCD synthesis, diamond powder is placed on top of aWC—Co substrate and the assembly is placed in a capsule and pressed athigh-pressure high-temperature. During pressing, the cobalt in the WC—Cosubstrate becomes molten and infiltrates the diamond powder, effectivelydissolving some of the outer surface of the diamond particles andprecipitating new diamond so that the diamond particles are stronglyconnected to each other by diamond intergrowth and strongly connected tothe WC—Co substrate by the solidified metal infiltrant. Adhesion of theintergrown PCD layer to the substrate therefore does not pose a problem.

However, the performance and lifetime of PCD are strongly dependent onmanaging the stresses in the PCD layer, ensuring thermal stability andobtaining the desired microstructure. These desirable properties may beachieved by choosing the appropriate means of combining the variousstarting materials during green body preparation prior to sintering athigh-pressure high-temperature. It has now surprisingly been found thatEPD may be used to produce a pre-sinter body comprising diamond or cBNdeposited on a substrate that is suitable for making a polycrystallinesuperhard material and elements comprising same.

Diamond grains typically have a negative charge and would tend tomigrate towards a positively charged electrode, the anode. Inembodiments of the method, the diamond particles or grains My beprovided with a positive charge and the substrate may be employed as acathode, in order to provide cathodic protection to the substrate, toassist in inhibiting the substrate from undergoing anodic corrosion,which could damage it and possibly make it unsuitable as a substrate fora polycrystalline diamond element. Accordingly, the diamond particles orgrains may be contacted with hydrogen or a source of hydrogen ions inorder to render them positively charged.

Electrophoretic deposition of diamond particles or grains, with orwithout additives, offers a simple, quick and controllable method forobtaining thin layers of uniform thickness and homogeneous compositionon flat or non-planar surfaces, and it is simple to vary the sequence ofthe layers according to the material property requirements for managingstresses in the PCD.

EXAMPLES

Embodiments are described in more detail with reference to the examplesbelow, which are not intended to be limiting.

Example 1

Diamond powder of 2.5 micron average particle size was treated in ahydrogen atmosphere at 800° C. for 1 hour in order to hydrogen-terminatethe surfaces of the diamond, thereby positively charging the diamondparticles.

An electrophoretic cell, similar to that depicted in FIG. 1, wasassembled, consisting of a platinum-coated titanium plate anode, and acathode consisting of a standard cylindrical cobalt-cemented tungstencarbide substrate as is normally used in high-pressure high-temperaturepolycrystalline diamond (PCD) synthesis. The substrate was approximately25 mm high, and had a diameter of approximately 20 mm. The substrate wasfitted with a close-fitting non-conducting rubber sleeve which blankedoff the sides and the back of the substrate, in order to prevent anydeposition of diamond particles on the sides of the cylinder. Electricalcontact was made through the rubber sleeve to the back of the substrate.

The electrodes were inserted into a glass beaker containing 200 mldeionised water with the exposed area of the substrate facing the Pt/Tianode, and the electrodes were electrically connected to a directcurrent power supply. The beaker was placed on a magnetic stirrer plateand the water was stirred vigorously by means of a magnetic stirrer bar.

Approximately 40 g of hydrogen-treated diamond was added to thedeionised water, resulting in an initial dispersed diamond concentrationof approximately 20%. The pH of the deionised water was adjusted withsmall aliquots of 15% nitric acid or 10% aqueous ammonium hydroxide inorder to maintain the pH in the range 2-4. The power supply was switchedon and a constant potential of approximately 3 V was applied forapproximately 2 minutes. A layer of diamond particles less thanapproximately 0.3 millimetres thick deposited on the Co—WC substratesurface. The layer was surprisingly well adherent to the substratesurface.

The substrate with diamond layer was placed in a capsule and sinteredunder standard high-pressure high-temperature conditions. SEM analysisof a cross-section of the sintered PCD showed a well-sintered layer ofapproximately 100 micron thick, with strong adherence to the Co—WCsubstrate.

Example 2

The procedure used in Example 1 was again followed, except that anadditional 40 g of hydrogen-terminated diamond powder of averageparticle size 12 micron was gradually added during the period of 2minutes when electrical potential was applied to the electrodes. A layerof diamond particles less than approximately 0.7 millimetres thickdeposited on the Co—WC substrate surface.

The substrate with diamond layer was placed in a capsule and sinteredunder standard high-pressure high-temperature conditions. SEM analysisof a cross-section of the sintered PCD, as shown in FIG. 3, showed awell-sintered layer of approximately 400 micron thick, with strongadherence to the Co—WC substrate. The average particle size of thediamond grains in the sintered PCD ranged from approximately 2.5 micronat the diamond-substrate interface to an average particle size ofapproximately 8 micron at the top surface of the PCD.

Example 3

The procedure used in Example 1 was again followed, except that thetotal duration of the applied potential lasted for approximately 10minutes. During this time, the electrode assembly was raised every 2minutes and lowered into a different solution. Two solutions were used:Solution 1 contained 40 g of hydrogen-treated diamond powder of averageparticle size 2.5 micron, and solution 2 contained 30 g ofhydrogen-treated diamond powder of average particle size 12 micron and10 g of hydrogen-treated diamond powder of average particle size 2.5micron. In this experiment, dispersion was achieved by also inserting anultrasonic probe to ultrasonically disperse the diamond powder. In thismanner, 5 layers of alternating average diamond particle size weredeposited, each layer less than approximately 0.3 millimetres thick.

The substrate with diamond layers was placed in a capsule and sinteredunder standard high-pressure high-temperature conditions. SEM analysisof a cross-section of the sintered PCD showed a well-sintered layer ofapproximately 1 millimetre thick, with strong adherence to the Co—WCsubstrate.

Example 4

The procedure used in Example 1 was again followed, except that thecathode was Co—WC substrate with a sintered PCD layer on top consistingof 12 micron average diamond grain size. Electrophoretic depositionresulted in a layer of diamond of average particle size 2.5 micron andless than approximately 0.3 millimetres thick on top of the sintered PCDlayer.

The substrate-PCD cutter with diamond layer deposited on top was placedin a capsule and sintered under standard high-pressure high-temperatureconditions. SEM analysis of a cross-section of the sintered PCD showed awell-sintered layer of approximately 100 micron thick of average diamondgrain size approximately 2.5 micron, with strong adherence to the PCDlayer consisting of 12 micron average diamond grain size.

Example 5

The same procedure was followed as in Example 1, except that the cellconfiguration was modified. The anode consisted of a platinum meshplaced horizontally at the bottom of the beaker, with the electricalconnection and lead covered in an electrically insulating rubber sleeve.The Co—WC cathode in its rubber sleeve was placed face downwards in theaqueous dispersion so that the substrate surface to be coated was facingthe anode. Care was taken to ensure that no gas bubbles were trappedagainst the Co—WC cathode when placed face downwards, as bubbles wouldprevent deposition, causing pinholes in the deposited layer. Anyagglomerates in the dispersion tended to settle towards the bottom ofthe beaker, falling through the mesh anode, and the resulting depositthat formed on the Co—WC cathode was of more uniform particle sizedistribution, without agglomerates being included in the depositedlayer. Repeated dip-and-coat in different solutions containing differentamounts of diamond powder, each of different particle size range,enabled the build-up of layers of diamond of varying thickness and grainsize.

1. A method for making polycrystalline superhard material, the methodcomprising providing an electrically conductive substrate defining atleast one deposition surface, electrophoretically depositing chargedsuperhard particles or grains on to the deposition surface(s) of thesubstrate to form a pre-sinter body, and subjecting the pre-sinter bodyto a temperature and pressure at which the superhard material isthermodynamically stable, sintering and forming polycrystallinesuperhard material.
 2. A method according to claim 1, wherein thesubstrate forms the cathode of an electrophoretic cell apparatus, thesuperhard particles or grains being suspended in a liquid in contactwith the deposition surface(s) and an anode, method further comprisingdepositing the superhard particles or grains on the depositionsurface(s) upon application of an electric potential between thesubstrate and the anode, the superhard particles or grains beingpositively charged so as to be so deposited.
 3. A method according toclaim 2, further comprising positioning the anode to define acomplementary surface or surfaces opposing the deposition surface(s) ofthe substrate.
 4. A method according to any one of the preceding claimsclaim 1, wherein the step of electrophoretically depositing chargedsuperhard particles or grains comprises depositing the superhardparticles or grains on the substrate in a series of layers or strata. 5.A method according to claim 1, wherein the deposition surface(s) of thesubstrate is/are masked in certain areas or regions, the step ofdepositing the superhard particles or grains comprising depositing thesuperhard particles or grains on exposed portions of the depositionsurface(s) so as to form discrete three dimensional polycrystallinesuperhard structures.
 6. A method according to claim 4, wherein the stepof electrophoretically depositing charged superhard particles or grainscomprises to form various layers or three dimensional structures ofpolycrystalline superhard material, comprises forming the layers orthree dimensional structures to have differing structuralcharacteristics from one another.
 7. A method according to claim 1,comprising forming a polycrystalline superhard material having asuperhard grain content of at least 80 percent and at most 98 percent ofthe volume of the polycrystalline superhard material.
 8. A methodaccording to claim 1, comprising forming polycrystalline diamondmaterial, the superhard particles or grains comprising diamond.
 9. Amethod according to claim 8, wherein the method comprises formingpolycrystalline diamond material comprising at most 10 volume percent ofa catalyst material for diamond.
 10. A method according to claim 1,wherein comprising forming a PCBN material, the superhard particles orgrains comprising cBN.
 11. A superhard wear element comprising apolycrystalline superhard material produced by a method as claimed inclaim
 1. 12. A superhard wear element as claimed in claim 11, comprisinga plurality of regions, each region comprising polycrystalline superhardmaterial having at least one different structural characteristic.
 13. Asuperhard wear element as claimed in claim 11, in which the superhardwear element is for use in machining, drilling or cutting a workpiececomprising metal.